Natural rubber, also called
IndiaIndia rubber or caoutchouc, as initially
produced, consists of polymers of the organic compound isoprene, with
minor impurities of other organic compounds, plus water.
MalaysiaMalaysia and
IndonesiaIndonesia are two of the leading rubber producers. Forms of
polyisoprene that are used as natural rubbers are classified as
elastomers.
Currently, rubber is harvested mainly in the form of the latex from
the rubber tree or others. The latex is a sticky, milky colloid drawn
off by making incisions in the bark and collecting the fluid in
vessels in a process called "tapping". The latex then is refined into
rubber ready for commercial processing. In major areas, latex is
allowed to coagulate in the collection cup. The coagulated lumps are
collected and processed into dry forms for marketing.
Natural rubberNatural rubber is used extensively in many applications and products,
either alone or in combination with other materials. In most of its
useful forms, it has a large stretch ratio and high resilience, and is
extremely waterproof.[1]

Varieties[edit]
HeveaHevea brasiliensis[edit]
The major commercial source of natural rubber latex is the Pará
rubber tree (
HeveaHevea brasiliensis), a member of the spurge family,
Euphorbiaceae. This species is preferred because it grows well under
cultivation. A properly managed tree responds to wounding by producing
more latex for several years.
Congo rubber[edit]
Congo rubber, formerly a major source of rubber, came from vines in
the genus
LandolphiaLandolphia (L. kirkii, L. heudelotis, and L. owariensis).[2]
Dandelion[edit]
DandelionDandelion milk contains latex. The latex exhibits the same quality as
the natural rubber from rubber trees. In the wild types of dandelion,
latex content is low and varies greatly. In Nazi Germany, research
projects tried to use dandelions as a base for rubber production, but
failed.[3] In 2013, by inhibiting one key enzyme and using modern
cultivation methods and optimization techniques, scientists in the
Fraunhofer Institute for Molecular Biology and Applied Ecology (IME)
in
GermanyGermany developed a cultivar that is suitable for commercial
production of natural rubber.[4] In collaboration with Continental
Tires, IME began a pilot facility.
Other[edit]
Many other plants produce forms of latex rich in isoprene polymers,
though not all produce usable forms of polymer as easily as the Pará.
Some of them require more elaborate processing to produce anything
like usable rubber, and most are more difficult to tap. Some produce
other desirable materials, for example gutta-percha (Palaquium
gutta)[5] and chicle from
ManilkaraManilkara species. Others that have been
commercially exploited, or at least showed promise as rubber sources,
include the rubber fig (Ficus elastica), Panama rubber tree (Castilla
elastica), various spurges (
EuphorbiaEuphorbia spp.), lettuce (Lactuca
species), the related Scorzonera tau-saghyz, various Taraxacum
species, including common dandelion (
TaraxacumTaraxacum officinale) and Russian
dandelion (
TaraxacumTaraxacum kok-saghyz), and perhaps most importantly for its
hypoallergenic properties, guayule (Parthenium argentatum). The term
gum rubber is sometimes applied to the tree-obtained version of
natural rubber in order to distinguish it from the synthetic
version.[1]
History[edit]
The first use of rubber was by the indigenous cultures of Mesoamerica.
The earliest archeological evidence of the use of natural latex from
the
Hevea treeHevea tree comes from the
OlmecOlmec culture, in which rubber was first
used for making balls for the
MesoamericanMesoamerican ballgame.
RubberRubber was later
used by the Maya and
AztecAztec cultures - in addition to making balls
Aztecs used rubber for other purposes such as making containers and to
make textiles waterproof by impregnating them with the latex
sap.[6][7]
The
Pará rubber treePará rubber tree is indigenous to South America. Charles Marie de
La Condamine is credited with introducing samples of rubber to the
Académie Royale des SciencesAcadémie Royale des Sciences of France in 1736.[8] In 1751, he
presented a paper by
François Fresneau to the Académie (published in
1755) that described many of rubber's properties. This has been
referred to as the first scientific paper on rubber.[8] In England,
Joseph Priestley, in 1770, observed that a piece of the material was
extremely good for rubbing off pencil marks on paper, hence the name
"rubber". It slowly made its way around England. In 1764 François
Fresnau discovered that turpentine was a rubber solvent. Giovanni
Fabbroni is credited with the discovery of naphtha as a rubber solvent
in 1779.
South AmericaSouth America remained the main source of the limited amounts of latex
rubber used during much of the 19th century. The trade was heavily
protected and exporting seeds from Brazil was a capital offense,
although no law prohibited it. Nevertheless, in 1876, Henry Wickham
smuggled 70,000
Pará rubber treePará rubber tree seeds from Brazil and delivered them
to Kew Gardens, England. Only 2,400 of these germinated. Seedlings
were then sent to India,
British CeylonBritish Ceylon (Sri Lanka), Dutch East Indies
(Indonesia), Singapore, and British Malaya. Malaya (now Peninsular
Malaysia) was later to become the biggest producer of rubber. In the
early 1900s, the
Congo Free StateCongo Free State in Africa was also a significant
source of natural rubber latex, mostly gathered by forced labor.
LiberiaLiberia and
NigeriaNigeria started production.
In
IndiaIndia , commercial cultivation was introduced by British planters,
although the experimental efforts to grow rubber on a commercial scale
were initiated as early as 1873 at the
CalcuttaCalcutta Botanical Gardens. The
first commercial
HeveaHevea plantations were established at Thattekadu in
KeralaKerala in 1902. In later years the plantation expanded to Karnataka,
Tamil NaduTamil Nadu and the
Andaman and Nicobar IslandsAndaman and Nicobar Islands of India.
IndiaIndia today
is the world's 3rd largest producer and 4th largest consumer.[9]
In
SingaporeSingapore and Malaya, commercial production was heavily promoted by
Sir Henry Nicholas Ridley, who served as the first Scientific Director
of the
SingaporeSingapore Botanic Gardens from 1888 to 1911. He distributed
rubber seeds to many planters and developed the first technique for
tapping trees for latex without causing serious harm to the tree.[10]
Because of his fervent promotion of this crop, he is popularly
remembered by the nickname "Mad Ridley".[11]
Pre-World War II[edit]
Charles GoodyearCharles Goodyear developed vulcanization in 1839, although
Mesoamericans used stabilized rubber for balls and other objects as
early as 1600 BC.[12][13]
Before World War II significant uses included door and window
profiles, hoses, belts, gaskets, matting, flooring and dampeners
(antivibration mounts) for the automotive industry. The use of rubber
in car tires (initially solid rather than pneumatic) in particular
consumed a significant amount of rubber.
GlovesGloves (medical, household
and industrial) and toy balloons were large consumers of rubber,
although the type of rubber used is concentrated latex. Significant
tonnage of rubber was used as adhesives in many manufacturing
industries and products, although the two most noticeable were the
paper and the carpet industries.
RubberRubber was commonly used to make
rubber bands and pencil erasers.
RubberRubber produced as a fiber, sometimes called 'elastic', had
significant value to the textile industry because of its excellent
elongation and recovery properties. For these purposes, manufactured
rubber fiber was made as either an extruded round fiber or rectangular
fibers cut into strips from extruded film. Because of its low dye
acceptance, feel and appearance, the rubber fiber was either covered
by yarn of another fiber or directly woven with other yarns into the
fabric.
RubberRubber yarns were used in foundation garments.
While rubber is still used in textile manufacturing, its low tenacity
limits its use in lightweight garments because latex lacks resistance
to oxidizing agents and is damaged by aging, sunlight, oil and
perspiration. The textile industry turned to neoprene (polymer of
chloroprene), a type of synthetic rubber, as well as another more
commonly used elastomer fiber, spandex (also known as elastane),
because of their superiority to rubber in both strength and
durability.
Properties[edit]

RubberRubber exhibits unique physical and chemical properties. Rubber's
stress–strain behavior exhibits the
Mullins effectMullins effect and the Payne
effect and is often modeled as hyperelastic.
RubberRubber strain
crystallizes.
Due to the presence of a double bond in each repeat unit, natural
rubber is susceptible to vulcanisation and sensitive to ozone
cracking.
The two main solvents for rubber are turpentine and naphtha
(petroleum). Because rubber does not dissolve easily, the material is
finely divided by shredding prior to its immersion.
An ammonia solution can be used to prevent the coagulation of raw
latex.
RubberRubber begins to melt at approximately 180 °C (356 °F).
Elasticity[edit]
Main article:
RubberRubber elasticity

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On a microscopic scale, relaxed rubber is a disorganized cluster of
erratically changing wrinkled chains. In stretched rubber, the chains
are almost linear. The restoring force is due to the preponderance of
wrinkled conformations over more linear ones. For the quantitative
treatment see ideal chain, for more examples see entropic force.
Cooling below the glass transition temperature permits local
conformational changes but a reordering is practically impossible
because of the larger energy barrier for the concerted movement of
longer chains. "Frozen" rubber's elasticity is low and strain results
from small changes of bond lengths and angles: this caused the
Challenger disaster, when the American Space Shuttle's flattened
o-rings failed to relax to fill a widening gap.[14] The glass
transition is fast and reversible: the force resumes on heating.
The parallel chains of stretched rubber are susceptible to
crystallization. This takes some time because turns of twisted chains
have to move out of the way of the growing crystallites.
Crystallization has occurred, for example, when, after days, an
inflated toy balloon is found withered at a relatively large remaining
volume. Where it is touched, it shrinks because the temperature of the
hand is enough to melt the crystals.
VulcanizationVulcanization of rubber creates disulfide bonds between chains, which
limits the degrees of freedom and results in chains that tighten more
quickly for a given strain, thereby increasing the elastic force
constant and making the rubber harder and less extensible.
Malodour[edit]
Raw rubber storage depots and rubber processing can produce malodour
that is serious enough to become a source of complaints and protest to
those living in the vicinity.[15]
Microbial impurities originate during the processing of block rubber.
These impurities break down during storage or thermal degradation and
produce volatile organic compounds. Examination of these compounds
using gas chromatography/mass spectrometry (GC/MS) and gas
chromatography (GC) indicates that they contain sulphur, ammonia,
alkenes, ketones, esters, hydrogen sulphite, nitrogen, and low
molecular weight fatty acids (C2-C5).[16][17]
When latex concentrate is produced from rubber, sulphuric acid is used
for coagulation. This produces malodourous hydrogen sulphide.[17]
The industry can mitigate these bad odours with scrubber systems.[17]
Chemical makeup[edit]

Chemical structure of cis-polyisoprene, the main constituent of
natural rubber. Synthetic cis-polyisoprene and natural
cis-polyisoprene are derived from different precursors, isopentenyl
pyrophosphate and isoprene.

LatexLatex is the polymer cis-1,4-polyisoprene – with a molecular weight
of 100,000 to 1,000,000 daltons. Typically, a small percentage (up to
5% of dry mass) of other materials, such as proteins, fatty acids,
resins, and inorganic materials (salts) are found in natural rubber.
PolyisoprenePolyisoprene can also be created synthetically, producing what is
sometimes referred to as "synthetic natural rubber", but the synthetic
and natural routes are different.[1] Some natural rubber sources, such
as gutta-percha, are composed of trans-1,4-polyisoprene, a structural
isomer that has similar properties.
Natural rubberNatural rubber is an elastomer and a thermoplastic. Once the rubber is
vulcanized, it turns into a thermoset. Most rubber in everyday use is
vulcanized to a point where it shares properties of both; i.e., if it
is heated and cooled, it is degraded but not destroyed.
The final properties of a rubber item depend not just on the polymer,
but also on modifiers and fillers, such as carbon black, factice,
whiting and others.
Biosynthesis[edit]
RubberRubber particles are formed in the cytoplasm of specialized
latex-producing cells called laticifers within rubber plants.[18]
RubberRubber particles are surrounded by a single phospholipid membrane with
hydrophobic tails pointed inward. The membrane allows biosynthetic
proteins to be sequestered at the surface of the growing rubber
particle, which allows new monomeric units to be added from outside
the biomembrane, but within the lacticifer. The rubber particle is an
enzymatically active entity that contains three layers of material,
the rubber particle, a biomembrane and free monomeric units. The
biomembrane is held tightly to the rubber core due to the high
negative charge along the double bonds of the rubber polymer
backbone.[19] Free monomeric units and conjugated proteins make up the
outer layer. The rubber precursor is isopentenyl pyrophosphate (an
allylic compound), which elongates by Mg2+-dependent condensation by
the action of rubber transferase. The monomer adds to the
pyrophosphate end of the growing polymer.[20] The process displaces
the terminal high-energy pyrophosphate. The reaction produces a cis
polymer. The initiation step is catalyzed by prenyltransferase, which
converts three monomers of isopentenyl pyrophosphate into farnesyl
pyrophosphate.[21] The farnesyl pyrophosphate can bind to rubber
transferase to elongate a new rubber polymer.
The required isopentenyl pyrophosphate is obtained from the mevalonate
pathway, which derives from acetyl-CoA in the cytosol. In plants,
isoprene pyrophosphate can also be obtained from the
1-deox-D-xyulose-5-phosphate/2-C-methyl-D-erythritol-4-phosphate
pathway within plasmids.[22] The relative ratio of the farnesyl
pyrophosphate initiator unit and isoprenyl pyrophosphate elongation
monomer determines the rate of new particle synthesis versus
elongation of existing particles. Though rubber is known to be
produced by only one enzyme, extracts of latex host numerous small
molecular weight proteins with unknown function. The proteins possibly
serve as cofactors, as the synthetic rate decreases with complete
removal.[23]
Production[edit]

RubberRubber is generally cultivated in large plantations. The image shows a
coconut shell used in collecting latex, in plantations in Kerala,
India.

Close to 28 million tons of rubber were produced in 2013, of which
approximately 44% was natural. Since the bulk is synthetic, which is
derived from petroleum, the price of natural rubber is determined, to
a large extent, by the prevailing global price of crude
oil.[24][25][26] Asia was the main source of natural rubber,
accounting for about 94% of output in 2005. The three largest
producers, Thailand,
IndonesiaIndonesia (2.4 million tons)[27] and Malaysia,
together account for around 72% of all natural rubber production.
Natural rubberNatural rubber is not cultivated widely in its native continent of
South AmericaSouth America due to the existence of South American leaf blight, and
other natural predators.
Cultivation[edit]
RubberRubber latex is extracted from rubber trees. The economic life period
of rubber trees in plantations is around 32 years — up to 7
years of immature phase and about 25 years of productive phase.
The soil requirement is well-drained, weathered soil consisting of
laterite, lateritic types, sedimentary types, nonlateritic red or
alluvial soils.
The climatic conditions for optimum growth of rubber trees are:

Rainfall of around 250 centimetres (98 in) evenly distributed
without any marked dry season and with at least 100 rainy days per
year
Temperature range of about 20 to 34 °C, with a monthly mean of
25 to 28 °C
Atmospheric humidity of around 80%
About 2000 hours sunshine per year at the rate of six hours per day
throughout the year
Absence of strong winds

Many high-yielding clones have been developed for commercial planting.
These clones yield more than 2,000 kg of dry rubber per hectare
per year, under ideal conditions.
Collection[edit]

In places such as
KeralaKerala and
Sri LankaSri Lanka where coconuts are in
abundance, the half shell of coconut was used as the latex collection
container. Glazed pottery or aluminium or plastic cups became more
common in
KeralaKerala and other countries. The cups are supported by a wire
that encircles the tree. This wire incorporates a spring so it can
stretch as the tree grows. The latex is led into the cup by a
galvanised "spout" knocked into the bark. Tapping normally takes place
early in the morning, when the internal pressure of the tree is
highest. A good tapper can tap a tree every 20 seconds on a standard
half-spiral system, and a common daily "task" size is between 450 and
650 trees. Trees are usually tapped on alternate or third days,
although many variations in timing, length and number of cuts are
used. "Tappers would make a slash in the bark with a small hatchet.
These slanting cuts allowed latex to flow from ducts located on the
exterior or the inner layer of bark (cambium) of the tree. Since the
cambium controls the growth of the tree, growth stops if it is cut.
Thus, rubber tapping demanded accuracy, so that the incisions would
not be too many given the size of the tree, or too deep, which could
stunt its growth or kill it."[28]
It is usual to tap a pannel at least twice, sometimes three times,
during the tree's life. The economic life of the tree depends on how
well the tapping is carried out, as the critical factor is bark
consumption. A standard in
MalaysiaMalaysia for alternate daily tapping is
25 cm (vertical) bark consumption per year. The latex-containing
tubes in the bark ascend in a spiral to the right. For this reason,
tapping cuts usually ascend to the left to cut more tubes.
The trees drip latex for about four hours, stopping as latex
coagulates naturally on the tapping cut, thus blocking the latex tubes
in the bark. Tappers usually rest and have a meal after finishing
their tapping work, then start collecting the liquid "field latex" at
about midday.
Field coagula[edit]

Mixed field coagula.

Smallholder's lump at a remilling factory

The four types of field coagula are "cuplump", "treelace",
"smallholders' lump" and "earth scrap". Each has significantly
different properties.[29] Some trees continue to drip after the
collection leading to a small amount of "cup lump" that is collected
at the next tapping. The latex that coagulates on the cut is also
collected as "tree lace". Tree lace and cup lump together account for
10–20% of the dry rubber produced.
LatexLatex that drips onto the ground,
"earth scrap", is also collected periodically for processing of
low-grade product.
Cup lump[edit]
Cup lump is the coagulated material found in the collection cup when
the tapper next visits the tree to tap it again. It arises from latex
clinging to the walls of the cup after the latex was last poured into
the bucket, and from late-dripping latex exuded before the
latex-carrying vessels of the tree become blocked. It is of higher
purity and of greater value than the other three types.
Tree lace[edit]
Tree lace is the coagulum strip that the tapper peels off the previous
cut before making a new cut. It usually has higher copper and
manganese contents than cup lump. Both copper and manganese are
pro-oxidants and can damage the physical properties of the dry rubber.
Smallholders' lump[edit]
Smallholders' lump is produced by smallholders who collect rubber from
trees far from the nearest factory. Many Indonesian smallholders, who
farm paddies in remote areas, tap dispersed trees on their way to work
in the paddy fields and collect the latex (or the coagulated latex) on
their way home. As it is often impossible to preserve the latex
sufficiently to get it to a factory that processes latex in time for
it to be used to make high quality products, and as the latex would
anyway have coagulated by the time it reached the factory, the
smallholder will coagulate it by any means available, in any container
available. Some smallholders use small containers, buckets etc., but
often the latex is coagulated in holes in the ground, which are
usually lined with plastic sheeting. Acidic materials and fermented
fruit juices are used to coagulate the latex — a form of
assisted biological coagulation. Little care is taken to exclude
twigs, leaves, and even bark from the lumps that are formed, which may
also include tree lace.
Earth scrap[edit]
Earth scrap is material that gathers around the base of the tree. It
arises from latex overflowing from the cut and running down the bark,
from rain flooding a collection cup containing latex, and from
spillage from tappers' buckets during collection. It contains soil and
other contaminants, and has variable rubber content, depending on the
amount of contaminants. Earth scrap is collected by field workers two
or three times a year and may be cleaned in a scrap-washer to recover
the rubber, or sold to a contractor who cleans it and recovers the
rubber. It is of low quality.
Processing[edit]

Removing coagulum from coagulating troughs.

LatexLatex coagulates in the cups if kept for long and must be collected
before this happens. The collected latex, "field latex", is
transferred into coagulation tanks for the preparation of dry rubber
or transferred into air-tight containers with sieving for ammoniation.
Ammoniation preserves the latex in a colloidal state for longer
periods of time.
LatexLatex is generally processed into either latex concentrate for
manufacture of dipped goods or coagulated under controlled, clean
conditions using formic acid. The coagulated latex can then be
processed into the higher-grade, technically specified block rubbers
such as SVR 3L or SVR CV or used to produce Ribbed Smoke Sheet grades.
Naturally coagulated rubber (cup lump) is used in the manufacture of
TSR10 and TSR20 grade rubbers. Processing for these grades is a size
reduction and cleaning process to remove contamination and prepare the
material for the final stage of drying.[30]
The dried material is then baled and palletized for storage and
shipment.
Vulcanized rubber[edit]

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Main article: Vulcanization

Torn latex rubber dry suit wrist seal

Natural rubberNatural rubber is often vulcanized, a process by which the rubber is
heated and sulfur, peroxide or bisphenol are added to improve
resistance and elasticity and to prevent it from perishing. Before
World War II, carbon black was often used as an additive to rubber to
improve its strength, especially in vehicle tires.
Transportation[edit]
Natural rubberNatural rubber latex is shipped from factories in south-west Asia,
South America, and West and Center Africa to destinations around the
world. As the cost of natural rubber has risen significantly and
rubber products are dense, the shipping methods offering the lowest
cost per unit weight are preferred. Depending on destination,
warehouse availability, and transportation conditions, some methods
are preferred by certain buyers. In international trade, latex rubber
is mostly shipped in 20-foot ocean containers. Inside the container,
smaller containers are used to store the latex.[31]
Uses[edit]

Compression molded (cured) rubber boots before the flashes are removed

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Uncured rubber is used for cements;[32] for adhesive, insulating, and
friction tapes; and for crepe rubber used in insulating blankets and
footwear. Vulcanized rubber has many more applications. Resistance to
abrasion makes softer kinds of rubber valuable for the treads of
vehicle tires and conveyor belts, and makes hard rubber valuable for
pump housings and piping used in the handling of abrasive sludge.
The flexibility of rubber is appealing in hoses, tires and rollers for
devices ranging from domestic clothes wringers to printing presses;
its elasticity makes it suitable for various kinds of shock absorbers
and for specialized machinery mountings designed to reduce vibration.
Its relative gas impermeability makes it useful in the manufacture of
articles such as air hoses, balloons, balls and cushions. The
resistance of rubber to water and to the action of most fluid
chemicals has led to its use in rainwear, diving gear, and chemical
and medicinal tubing, and as a lining for storage tanks, processing
equipment and railroad tank cars. Because of their electrical
resistance, soft rubber goods are used as insulation and for
protective gloves, shoes and blankets; hard rubber is used for
articles such as telephone housings, parts for radio sets, meters and
other electrical instruments. The coefficient of friction of rubber,
which is high on dry surfaces and low on wet surfaces, leads to its
use for power-transmission belting and for water-lubricated bearings
in deep-well pumps. Indian rubber balls or lacrosse balls are made of
rubber.
Around 25 million tonnes of rubber are produced each year, of which 30
percent is natural.[33] The remainder is synthetic rubber derived from
petrochemical sources. The top end of latex production results in
latex products such as surgeons' gloves, condoms, balloons and other
relatively high-value products. The mid-range which comes from the
technically specified natural rubber materials ends up largely in
tires but also in conveyor belts, marine products, windshield wipers
and miscellaneous goods.
Natural rubberNatural rubber offers good elasticity, while
synthetic materials tend to offer better resistance to environmental
factors such as oils, temperature, chemicals and ultraviolet light.
"Cured rubber" is rubber that has been compounded and subjected to the
vulcanisation process to create cross-links within the rubber matrix.
Allergic reactions[edit]
Main article:
LatexLatex allergy
Some people have a serious latex allergy, and exposure to natural
latex rubber products such as latex gloves can cause anaphylactic
shock. The antigenic proteins found in
HeveaHevea latex may be deliberately
reduced (though not eliminated)[34] through processing.
LatexLatex from non-
HeveaHevea sources, such as Guayule, can be used without
allergic reaction by persons with an allergy to
HeveaHevea latex.[35]
Some allergic reactions are not to the latex itself, but from residues
of chemicals used to accelerate the cross-linking process. Although
this may be confused with an allergy to latex, it is distinct from it,
typically taking the form of
Type IV hypersensitivity in the presence
of traces of specific processing chemicals.[34][36]
Microbial degradation[edit]
Natural rubberNatural rubber is susceptible to degradation by a wide range of
bacteria[which?].[37][38][39][40][41][42][43][44] The bacteria
Streptomyces coelicolor, Pseudomonas citronellolis, and
NocardiaNocardia spp.
are capable of degrading vulcanized natural rubber.[45]
See also[edit]